1 CPU hotplug Support in Linux(tm) Kernel
2 3 Maintainers:
4 CPU Hotplug Core:
5 Rusty Russell <rusty@rustcorp.com.au>
6 Srivatsa Vaddagiri <vatsa@in.ibm.com>
7 i386:
8 Zwane Mwaikambo <zwane@arm.linux.org.uk>
9 ppc64:
10 Nathan Lynch <nathanl@austin.ibm.com>
11 Joel Schopp <jschopp@austin.ibm.com>
12 ia64/x86_64:
13 Ashok Raj <ashok.raj@intel.com>
14 s390:
15 Heiko Carstens <heiko.carstens@de.ibm.com>
16 17Authors: Ashok Raj <ashok.raj@intel.com>
18Lots of feedback: Nathan Lynch <nathanl@austin.ibm.com>,
19 Joel Schopp <jschopp@austin.ibm.com>
20 21Introduction
22 23Modern advances in system architectures have introduced advanced error
24reporting and correction capabilities in processors. CPU architectures permit
25partitioning support, where compute resources of a single CPU could be made
26available to virtual machine environments. There are couple OEMS that
27support NUMA hardware which are hot pluggable as well, where physical
28node insertion and removal require support for CPU hotplug.
29 30Such advances require CPUs available to a kernel to be removed either for
31provisioning reasons, or for RAS purposes to keep an offending CPU off
32system execution path. Hence the need for CPU hotplug support in the
33Linux kernel.
34 35A more novel use of CPU-hotplug support is its use today in suspend
36resume support for SMP. Dual-core and HT support makes even
37a laptop run SMP kernels which didn't support these methods. SMP support
38for suspend/resume is a work in progress.
39 40General Stuff about CPU Hotplug
41--------------------------------
42 43Command Line Switches
44---------------------
45maxcpus=n Restrict boot time cpus to n. Say if you have 4 cpus, using
46 maxcpus=2 will only boot 2. You can choose to bring the
47 other cpus later online, read FAQ's for more info.
48 49additional_cpus=n (*) Use this to limit hotpluggable cpus. This option sets
50 cpu_possible_mask = cpu_present_mask + additional_cpus
51 52cede_offline={"off","on"} Use this option to disable/enable putting offlined
53 processors to an extended H_CEDE state on
54 supported pseries platforms.
55 If nothing is specified,
56 cede_offline is set to "on".
57 58(*) Option valid only for following architectures
59- ia64
60 61ia64 uses the number of disabled local apics in ACPI tables MADT to
62determine the number of potentially hot-pluggable cpus. The implementation
63should only rely on this to count the # of cpus, but *MUST* not rely
64on the apicid values in those tables for disabled apics. In the event
65BIOS doesn't mark such hot-pluggable cpus as disabled entries, one could
66use this parameter "additional_cpus=x" to represent those cpus in the
67cpu_possible_mask.
68 69possible_cpus=n [s390,x86_64] use this to set hotpluggable cpus.
70 This option sets possible_cpus bits in
71 cpu_possible_mask. Thus keeping the numbers of bits set
72 constant even if the machine gets rebooted.
73 74CPU maps and such
75-----------------
76[More on cpumaps and primitive to manipulate, please check
77include/linux/cpumask.h that has more descriptive text.]
78 79cpu_possible_mask: Bitmap of possible CPUs that can ever be available in the
80system. This is used to allocate some boot time memory for per_cpu variables
81that aren't designed to grow/shrink as CPUs are made available or removed.
82Once set during boot time discovery phase, the map is static, i.e no bits
83are added or removed anytime. Trimming it accurately for your system needs
84upfront can save some boot time memory. See below for how we use heuristics
85in x86_64 case to keep this under check.
86 87cpu_online_mask: Bitmap of all CPUs currently online. Its set in __cpu_up()
88after a cpu is available for kernel scheduling and ready to receive
89interrupts from devices. Its cleared when a cpu is brought down using
90__cpu_disable(), before which all OS services including interrupts are
91migrated to another target CPU.
92 93cpu_present_mask: Bitmap of CPUs currently present in the system. Not all
94of them may be online. When physical hotplug is processed by the relevant
95subsystem (e.g ACPI) can change and new bit either be added or removed
96from the map depending on the event is hot-add/hot-remove. There are currently
97no locking rules as of now. Typical usage is to init topology during boot,
98at which time hotplug is disabled.
99 100You really dont need to manipulate any of the system cpu maps. They should
101be read-only for most use. When setting up per-cpu resources almost always use
102cpu_possible_mask/for_each_possible_cpu() to iterate.
103 104Never use anything other than cpumask_t to represent bitmap of CPUs.
105 106 #include <linux/cpumask.h>
107 108 for_each_possible_cpu - Iterate over cpu_possible_mask
109 for_each_online_cpu - Iterate over cpu_online_mask
110 for_each_present_cpu - Iterate over cpu_present_mask
111 for_each_cpu_mask(x,mask) - Iterate over some random collection of cpu mask.
112 113 #include <linux/cpu.h>
114 get_online_cpus() and put_online_cpus():
115 116The above calls are used to inhibit cpu hotplug operations. While the
117cpu_hotplug.refcount is non zero, the cpu_online_mask will not change.
118If you merely need to avoid cpus going away, you could also use
119preempt_disable() and preempt_enable() for those sections.
120Just remember the critical section cannot call any
121function that can sleep or schedule this process away. The preempt_disable()
122will work as long as stop_machine_run() is used to take a cpu down.
123 124CPU Hotplug - Frequently Asked Questions.
125 126Q: How to enable my kernel to support CPU hotplug?
127A: When doing make defconfig, Enable CPU hotplug support
128 129 "Processor type and Features" -> Support for Hotpluggable CPUs
130 131Make sure that you have CONFIG_HOTPLUG, and CONFIG_SMP turned on as well.
132 133You would need to enable CONFIG_HOTPLUG_CPU for SMP suspend/resume support
134as well.
135 136Q: What architectures support CPU hotplug?
137A: As of 2.6.14, the following architectures support CPU hotplug.
138 139i386 (Intel), ppc, ppc64, parisc, s390, ia64 and x86_64
140 141Q: How to test if hotplug is supported on the newly built kernel?
142A: You should now notice an entry in sysfs.
143 144Check if sysfs is mounted, using the "mount" command. You should notice
145an entry as shown below in the output.
146 147 ....
148 none on /sys type sysfs (rw)
149 ....
150 151If this is not mounted, do the following.
152 153 #mkdir /sysfs
154 #mount -t sysfs sys /sys
155 156Now you should see entries for all present cpu, the following is an example
157in a 8-way system.
158 159 #pwd
160 #/sys/devices/system/cpu
161 #ls -l
162 total 0
163 drwxr-xr-x 10 root root 0 Sep 19 07:44 .
164 drwxr-xr-x 13 root root 0 Sep 19 07:45 ..
165 drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu0
166 drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu1
167 drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu2
168 drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu3
169 drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu4
170 drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu5
171 drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu6
172 drwxr-xr-x 3 root root 0 Sep 19 07:48 cpu7
173 174Under each directory you would find an "online" file which is the control
175file to logically online/offline a processor.
176 177Q: Does hot-add/hot-remove refer to physical add/remove of cpus?
178A: The usage of hot-add/remove may not be very consistently used in the code.
179CONFIG_HOTPLUG_CPU enables logical online/offline capability in the kernel.
180To support physical addition/removal, one would need some BIOS hooks and
181the platform should have something like an attention button in PCI hotplug.
182CONFIG_ACPI_HOTPLUG_CPU enables ACPI support for physical add/remove of CPUs.
183 184Q: How do i logically offline a CPU?
185A: Do the following.
186 187 #echo 0 > /sys/devices/system/cpu/cpuX/online
188 189Once the logical offline is successful, check
190 191 #cat /proc/interrupts
192 193You should now not see the CPU that you removed. Also online file will report
194the state as 0 when a cpu if offline and 1 when its online.
195 196 #To display the current cpu state.
197 #cat /sys/devices/system/cpu/cpuX/online
198 199Q: Why can't i remove CPU0 on some systems?
200A: Some architectures may have some special dependency on a certain CPU.
201 202For e.g in IA64 platforms we have ability to sent platform interrupts to the
203OS. a.k.a Corrected Platform Error Interrupts (CPEI). In current ACPI
204specifications, we didn't have a way to change the target CPU. Hence if the
205current ACPI version doesn't support such re-direction, we disable that CPU
206by making it not-removable.
207 208In such cases you will also notice that the online file is missing under cpu0.
209 210Q: How do i find out if a particular CPU is not removable?
211A: Depending on the implementation, some architectures may show this by the
212absence of the "online" file. This is done if it can be determined ahead of
213time that this CPU cannot be removed.
214 215In some situations, this can be a run time check, i.e if you try to remove the
216last CPU, this will not be permitted. You can find such failures by
217investigating the return value of the "echo" command.
218 219Q: What happens when a CPU is being logically offlined?
220A: The following happen, listed in no particular order :-)
221 222- A notification is sent to in-kernel registered modules by sending an event
223 CPU_DOWN_PREPARE or CPU_DOWN_PREPARE_FROZEN, depending on whether or not the
224 CPU is being offlined while tasks are frozen due to a suspend operation in
225 progress
226- All processes are migrated away from this outgoing CPU to new CPUs.
227 The new CPU is chosen from each process' current cpuset, which may be
228 a subset of all online CPUs.
229- All interrupts targeted to this CPU is migrated to a new CPU
230- timers/bottom half/task lets are also migrated to a new CPU
231- Once all services are migrated, kernel calls an arch specific routine
232 __cpu_disable() to perform arch specific cleanup.
233- Once this is successful, an event for successful cleanup is sent by an event
234 CPU_DEAD (or CPU_DEAD_FROZEN if tasks are frozen due to a suspend while the
235 CPU is being offlined).
236 237 "It is expected that each service cleans up when the CPU_DOWN_PREPARE
238 notifier is called, when CPU_DEAD is called its expected there is nothing
239 running on behalf of this CPU that was offlined"
240 241Q: If i have some kernel code that needs to be aware of CPU arrival and
242 departure, how to i arrange for proper notification?
243A: This is what you would need in your kernel code to receive notifications.
244 245 #include <linux/cpu.h>
246 static int __cpuinit foobar_cpu_callback(struct notifier_block *nfb,
247 unsigned long action, void *hcpu)
248 {
249 unsigned int cpu = (unsigned long)hcpu;
250 251 switch (action) {
252 case CPU_ONLINE:
253 case CPU_ONLINE_FROZEN:
254 foobar_online_action(cpu);
255 break;
256 case CPU_DEAD:
257 case CPU_DEAD_FROZEN:
258 foobar_dead_action(cpu);
259 break;
260 }
261 return NOTIFY_OK;
262 }
263 264 static struct notifier_block __cpuinitdata foobar_cpu_notifer =
265 {
266 .notifier_call = foobar_cpu_callback,
267 };
268 269You need to call register_cpu_notifier() from your init function.
270Init functions could be of two types:
2711. early init (init function called when only the boot processor is online).
2722. late init (init function called _after_ all the CPUs are online).
273 274For the first case, you should add the following to your init function
275 276 register_cpu_notifier(&foobar_cpu_notifier);
277 278For the second case, you should add the following to your init function
279 280 register_hotcpu_notifier(&foobar_cpu_notifier);
281 282You can fail PREPARE notifiers if something doesn't work to prepare resources.
283This will stop the activity and send a following CANCELED event back.
284 285CPU_DEAD should not be failed, its just a goodness indication, but bad
286things will happen if a notifier in path sent a BAD notify code.
287 288Q: I don't see my action being called for all CPUs already up and running?
289A: Yes, CPU notifiers are called only when new CPUs are on-lined or offlined.
290 If you need to perform some action for each cpu already in the system, then
291 292 for_each_online_cpu(i) {
293 foobar_cpu_callback(&foobar_cpu_notifier, CPU_UP_PREPARE, i);
294 foobar_cpu_callback(&foobar_cpu_notifier, CPU_ONLINE, i);
295 }
296 297Q: If i would like to develop cpu hotplug support for a new architecture,
298 what do i need at a minimum?
299A: The following are what is required for CPU hotplug infrastructure to work
300 correctly.
301 302 - Make sure you have an entry in Kconfig to enable CONFIG_HOTPLUG_CPU
303 - __cpu_up() - Arch interface to bring up a CPU
304 - __cpu_disable() - Arch interface to shutdown a CPU, no more interrupts
305 can be handled by the kernel after the routine
306 returns. Including local APIC timers etc are
307 shutdown.
308 - __cpu_die() - This actually supposed to ensure death of the CPU.
309 Actually look at some example code in other arch
310 that implement CPU hotplug. The processor is taken
311 down from the idle() loop for that specific
312 architecture. __cpu_die() typically waits for some
313 per_cpu state to be set, to ensure the processor
314 dead routine is called to be sure positively.
315 316Q: I need to ensure that a particular cpu is not removed when there is some
317 work specific to this cpu is in progress.
318A: There are two ways. If your code can be run in interrupt context, use
319 smp_call_function_single(), otherwise use work_on_cpu(). Note that
320 work_on_cpu() is slow, and can fail due to out of memory:
321 322 int my_func_on_cpu(int cpu)
323 {
324 int err;
325 get_online_cpus();
326 if (!cpu_online(cpu))
327 err = -EINVAL;
328 else
329#if NEEDS_BLOCKING
330 err = work_on_cpu(cpu, __my_func_on_cpu, NULL);
331#else
332 smp_call_function_single(cpu, __my_func_on_cpu, &err,
333 true);
334#endif
335 put_online_cpus();
336 return err;
337 }
338 339Q: How do we determine how many CPUs are available for hotplug.
340A: There is no clear spec defined way from ACPI that can give us that
341 information today. Based on some input from Natalie of Unisys,
342 that the ACPI MADT (Multiple APIC Description Tables) marks those possible
343 CPUs in a system with disabled status.
344 345 Andi implemented some simple heuristics that count the number of disabled
346 CPUs in MADT as hotpluggable CPUS. In the case there are no disabled CPUS
347 we assume 1/2 the number of CPUs currently present can be hotplugged.
348 349 Caveat: Today's ACPI MADT can only provide 256 entries since the apicid field
350 in MADT is only 8 bits.
351 352User Space Notification
353 354Hotplug support for devices is common in Linux today. Its being used today to
355support automatic configuration of network, usb and pci devices. A hotplug
356event can be used to invoke an agent script to perform the configuration task.
357 358You can add /etc/hotplug/cpu.agent to handle hotplug notification user space
359scripts.
360 361 #!/bin/bash
362 # $Id: cpu.agent
363 # Kernel hotplug params include:
364 #ACTION=%s [online or offline]
365 #DEVPATH=%s
366 #
367 cd /etc/hotplug
368 . ./hotplug.functions
369 370 case $ACTION in
371 online)
372 echo `date` ":cpu.agent" add cpu >> /tmp/hotplug.txt
373 ;;
374 offline)
375 echo `date` ":cpu.agent" remove cpu >>/tmp/hotplug.txt
376 ;;
377 *)
378 debug_mesg CPU $ACTION event not supported
379 exit 1
380 ;;
381 esac
382